Conventional motion damping devices, such as shock absorbers, typically use motors, solenoids and other moving parts to control the flow of a damping liquid (such as oil) within the device. These components must be small, operate within a very harsh environment and must be very reliable.
More recently, the manufacturers of some motion-damping devices have proposed to control damping through the use of an electrorheological fluid whose viscosity can be controlled electrically. The properties of such fluids and their electrical control are now well known. See, for example, U.S. Pat. Nos. 4,733,758 and 4,819,772.
The latter type of motion damping devices still suffer from certain disadvantages, including undesirable complexity. Further, and especially in the case of an automotive shock absorber, these devices do not have the ability to easily indicate the position of the moveable piston which is normally included in any shock absorber. Knowledge of the position of the piston is particularly useful in connection with active (electronically controlled) suspension systems.